1. Field of the Invention
The present invention relates to chip type piezoelectric filters having a circuit including a plurality of electrically coupled piezoelectric filter sections, and more particularly, to a chip type piezoelectric filter having a circuit including a first energy trap type piezoelectric filter section and a second energy trap type piezoelectric filter section which are electrically connected to each other via a coupling capacitor.
2. Description of the Prior Art
Conventionally, energy trap type piezoelectric filters have been used as intermediate frequency filters in portable or handheld telephones functioning as mobile telecommunication devices. In the piezoelectric filters of this type, it is required that such filters be arranged as surface-mountable chip components.
For such a purpose, a chip type piezoelectric filter has been proposed which is shown in an exploded perspective view in FIG. 19. As apparent from FIG. 19, a chip type piezoelectric filter 51 includes a piezoelectric substrate 52 which is made of a piezoelectric material, such as lead zirconate titanate (PZT) piezoelectric ceramics. First and second filter sections and a coupling capacitance section are provided on the piezoelectric substrate 52.
More specifically, the first filter section includes a pair of resonance electrodes 53a, 53b disposed on the upper or top surface of the piezoelectric substrate 52. A common electrode (not show) is provided on the lower or bottom surface of the piezoelectric substrate 52. The common electrode on the bottom surface of the substrate 52 opposes the resonance electrodes 53a, 53b such that the resonance electrodes are located on the top surface of substrate 52 whereas the common electrode is on the bottom surface of the substrate 52 to define a "top/bottom-surface overlap" relationship. Similarly, the second filter section includes a pair of resonance electrodes 54a, 54b located on the top surface of piezoelectric substrate 52 while a common electrode (not shown) is disposed on the bottom surface of the substrate 52.
The resonance electrode 53a is connected to a lead electrode 55a which extends along one end portion of the piezoelectric substrate 52. The resonance electrode 54a is connected to a lead electrode 55b extending along the opposite end portion of piezoelectric substrate 52. The lead electrodes 55a, 55b are connected respectively to outer or external electrodes each serving as an input electrode or an output electrode, as will be described later. A capacitance electrode 56a is disposed at a central portion on the top surface of the piezoelectric substrate 52. Similarly, a capacitance electrode of substantially the same size as electrode 56a is disposed on the bottom surface of the substrate 52 in a way such that it opposes the capacitance electrode 56a with the substrate 52 being "sandwiched" therebetween. The capacitance electrode on the bottom surface of the substrate 52 extends up to one side edge of the substrate 52 (for connection with an external electrode 58b on the edge portion). The pair of capacitance electrodes define a capacitor which functions as a coupling capacitance section.
The capacitance electrode 56a is connected to the resonance electrodes 53b, 54b at an edge portion of the top surface of the substrate 52. The capacitance electrode 56a is elongated so as to be located at one long-edge side surface of the piezoelectric substrate 52. A pair of outer substrates 57, 58 are mounted on the opposite surfaces of the piezoelectric substrate 52 and adhered to the substrate 52 via a suitable adhesive. Recess portions 58d (one of which located on the substrate 57 side is not visible in the figure) are formed in the surfaces of outer substrates 57, 58 to form spaces for facilitating vibration of the first and second filter sections. Alternatively, such adhesive used may be coated on selected surface areas of the piezoelectric substrate 52 other than those areas in which the first and second filter sections are located. In another alternative arrangement, more than one recess portion may be formed in the opposing surfaces of the outer substrates 57, 58 and the adhesive may be coated on the inner surfaces of outer substrates 57, 58.
The outer substrate 57 includes external electrodes 57a to 57c. The external electrodes 57a-57c are formed in such a way that each of them extends across the top surface of external substrate 57 to be located along on a pair of opposite long-side edge surfaces thereof. Similarly, the outer substrate 58 includes external electrodes 58a-58c. The external electrodes 58a-58c are formed so that each extends across the bottom surface of external substrate 58 to extend along a pair of long-side edge surfaces thereof.
After lamination, each of the external electrodes 57a-57c is electrically connected to a corresponding one of the external electrodes 58a-58c. Accordingly, the external electrodes 57a, 58a are connected to the lead electrode 55 a for use as the input electrode. Also, the external electrodes 57b, 58b are connected to the capacitance electrode disposed on the bottom surface of the piezoelectric substrate 52. Additionally, the external electrodes 57b, 58b are coupled to ground.
The external electrodes 57c, 58c are electrically connected together and are further connected to the lead electrode 55b for use as the output electrode.
A circuit configuration of the chip type piezoelectric filter 51 arranged as described above is shown in FIG. 20. As apparent from FIG. 20, the chip type piezoelectric filter 51 has a circuit configuration in which the first and second filter sections 53, 54 are connected to each other via the coupling capacitance section 56.
In such a chip type piezoelectric filter 51, the first and second filter sections 53, 54 and its related coupling capacitance section 56 are located on a single piezoelectric substrate 52. As a result of this arrangement, when the filter 51 is surface mounted using external electrodes 57a-58c, the resulting mount space required increases undesirably.
One technique for reducing the surface mount area is to decrease the areas of the resonance electrodes 53a, 53b, 54a, 54b and common electrodes (not shown). However, when the electrode-to-electrode opposing area decreases in the filter section, the electrostatic capacitance decreases in value, causing an increase in the value of the matching impedance.
Furthermore, in cases where the size of the piezoelectric substrate 52 is decreased while the electrode areas are unchanged with respect to the first and second filter sections, the resultant filter characteristics are deteriorated due to the fact that vibration of the first and second filter sections is hindered or suppressed at a portion which is laminated by the adhesive used for connection of the elements.
Accordingly, it has not been possible to decrease the size of such a filter by simply decreasing the electrode area and the dimensions of piezoelectric substrate 52.